Prostaglandin D2 (PGD2) has long been associated with inflammatory and atopic conditions, specifically allergic diseases such as asthma, rhinitis and atopic dermatitis (Lewis et al. (1982) J. Immunol. 129, 1627). PGD2 belongs to a class of compounds derived from the 20-carbon fatty acid skeleton of arachidonic acid. In response to an antigen challenge, PGD2 is released in large amounts into the airway as well as to the skin during an acute allergic response. The DP receptor, which is a member of the G-protein coupled receptor (GPCRo) subfamily, has long been thought to be the only receptor of PGD2. DP's role in allergic asthma has been demonstrated with DP deficient mice (Matsuoka et al. (2000) Science 287, 2013-2017). However, despite intense interest in the role of PGD2 in the inflammatory response, a direct link between DP receptor activation and PGD2-stimulated eosinophil migration has not been established (Woodward et al. (1990) Invest. Ophthalomol. Vis. Sci. 31, 138-146; Woodward et al. (1993) Eur. J. Pharmacol. 230, 327-333).
More recently, another G-protein coupled receptor, referred to as “Chemoattractant Receptor-Homologous molecule expressed on T-Helper 2 cells” (CRTH2) (Nagata et al. (1999) J. Immunol. 162, 1278-1286, Hirai et al. (2001) J. Exp. Med. 193, 255-261) has recently been identified as a receptor for PGD2 and this discovery has begun to shed light on the mechanism of action of PGD2. CRTH2, which is also referred to as DP2, GPR44 or DLIR, shows little structural similarity with the DP receptor and other prostanoid receptors. However, CRTH2 possesses similar affinity for PGD2. Among peripheral blood T lymphocytes, human CRTH2 is selectively expressed on Th2 cells and is highly expressed on cell types associated with allergic inflammation such as eosinophils, basophiles and Th2 cells. In addition, CRTH2 mediates PGD2 dependent cell migration of blood eosinophils and basophiles. Furthermore, increased numbers of circulating T cells expressing CRTH2 have been correlated with the severity of atopic dermatitis (Cosmi et al. (2000) Eur. J. Immunol. 30, 2972-2979). The interaction of CRTH2 with PGD2 plays a critical role in the allergen-induced recruitment of Th2 cells in the target tissues of allergic inflammation. Compounds that inhibit the binding of CRTH2 and PGD2 should therefore be useful for the treatment of allergic diseases.
Allergic disease, like asthma, and inflammatory dermatoses represent a major class of complex, and typically chronic, inflammatory diseases that currently affect about 10% of the population and that number appears to be increasing (Bush, R. K., Georgitis J. W., Handbook of asthma and rhinitis. 1st ed. (1997), Abingdon: Blackwell Science. 270). Atopic dermatitis is a chronic skin disease, wherein the skin becomes extremely itchy. It accounts for 10 to 20 percent of all visits to dermatologists. The increasing incidence of allergic diseases and inflammatory dermatoses worldwide underscores the need for new therapies to effectively treat or prevent these diseases. Currently, numerous classes of pharmaceutical agents are widely used to treat these diseases, for example, antihistamines, decongestants, anticholinergics, methylxanthines, cromolyns, corticosteroids, and leukotriene modulators. However, the usefulness of these agents is often limited by side effects and low efficacy.
It has been reported recently that 3-sulphur-substituted indole derivatives (A) exhibit CRTH2 activity (WO 04/106302, AstraZeneca AB) and are potentially useful for the treatment of various respiratory diseases.

WO 04/096777 (Bayer Healthcare AG) relates to pyrimidine derivatives, which are useful for the treatment of diseases mediated by CRTH2.

WO 04/035543 and WO 05/102338 (Warner-Lambert Company LLC) disclose tetrahydrochinoline derivatives as CRTH2 antagonists (C), which are also described to be effective in the treatment of neuropatic pain.

Specific tetrahydrochinoline derivatives as CRTH2 modulators are also provided by WO 04/032848 (Millennium Pharmaceutical Inc.) and WO 05/007094 (Tularik Inc.). These tetrahydrochinoline derivatives are said to be useful for treating disorders associated with allergic inflammation processes.